The present invention relates to internal combustion engines having axial flow rotary valves incorporating an inlet and an exhaust port in the same valve. Each valve rotates with small clearance in a bore in the cylinder head of the engine. The ports terminate as openings in the periphery of the valve. During rotation, these openings periodically communicate with a similar window in the cylinder head allowing the passage of gas from the valve to the cylinder and vice versa. The clearance between the valve and the bore is sealed by an array of floating seals preferably having two elongate axial seals adjacent opposite sides of the window, each located in a slot in the cylinder head bore. For combustion efficiency and packaging reasons, the spark plugs are typically adjacent the cylinder head bores and the axial seal slots, and in the case of a multi-cylinder engine, the spark plugs are between the cylinder head bores of adjacent cylinders. Multi-cylinder inline engines with each cylinder having a single axial flow rotary valve are arranged with the axis of each valve substantially perpendicular to the crankshaft axis. U.S. Pat. No. 4,852,532 (Bishop) discloses an axial flow rotary valve engine having these features.
All internal combustion engines require cooling of the cylinders and cylinder heads. In water or liquid cooled engines, the engine is typically connected to a radiator through two pipes. A bottom pipe is generally connected to a water pump mounted on or in the engine that pumps coolant through the engine and out through a top pipe back into the radiator.
The cylinder heads and blocks of liquid cooled internal combustion engines are typically machined from castings. The cooling passages are formed during the casting process using cores. The process and techniques of casting cylinder heads and blocks with cooling passages formed by cores is well known. However, the layout of multi-cylinder axial flow rotary valve engines presents difficulties in cooling the head that cannot be overcome by providing cast cooling passages and the solutions applicable to other types of rotary valve are not suited to axial flow rotary valves.
In any engine it is essential to identify the areas that are subjected to the greatest heat load and to provide adequate cooling to these areas. Prior art methods of cooling axial flow rotary valve engines with an array of floating seals have not adequately identified and addressed all of the cooling requirements of this type of engine. In an axial flow rotary valve engine there are four sources of significant heat input to the cylinder head.
Firstly, there are the axial gas sealing elements. The axial seals are subjected to high heat loads from the hot combustion gases. These combustion gases provide the pressure to energise the axial seals in a similar manner to a piston ring by pressing them against the circumferentially outermost sides of the axial slots, remote from the window, and against the valve itself. Heat is also transferred to the axial seals from the hot outside diameter of the valve and the only heat path available to cool the seals is through the sides of the axial slots remote from the window. The axial seal temperature must be maintained at a sufficiently low level to ensure that the oil used to lubricate the surface of the valve does not carbonise.
Secondly, there is the area surrounding the nose of the spark plug. The nose of the spark plug is defined as that area of the spark plug extending from the spark plug gap to the spark plug seat. In conventional spark plugs this area is coincident with the threaded portion of the spark plug. The nose of the spark plug is exposed directly to the hottest part of the combustion chamber as this is where combustion commences and is thus subject to heat transfer from the hot combustion gases for the entire combustion period. In conventional poppet valve engines this high localised heat input adjacent the spark plug nose is handled by having generous cast cooling passages around the spark plug boss adjacent the threaded portion of the spark plug mounting hole.
Thirdly, there is the portion of the combustion chamber surface immediately surrounding the end of the spark plug that is exposed to the combustion chamber. Just as the spark plug receives a very high localised heat input due to its location at the point of first combustion so does that portion of the combustion chamber surface immediately surrounding the end of the spark plug nose. A typical axial flow rotary valve engine has a section of the combustion chamber surface that extends from the nose of the spark plug, up into the window, to the bore in the cylinder head, which has a very small volume contained behind it. This section of the combustion chamber has a very large surface area to volume ratio and will consequently see a very high heat input and high temperature rises. This problem is compounded by the fact that this volume is located a long distance from any cooling passages in previous designs such as that shown in U.S. Pat. No. 4,852,532 (Bishop).
Finally, there are the circumferentially innermost surfaces and the floors of axial slots, which are exposed to the hot combustion gases that are used to energise the axial sealing elements. These surfaces put heat into the same small volume described above in relation to the third source of heat. This further increases the surface area to volume ratio of that portion of the head contained between the combustion chamber surface, the axial slot and the spark plug.
These four heat sources converge in the area contained between the spark plug, the bore in the cylinder head, the window and the combustion chamber. As a result this area is subject to very high localised heat loads and this area is the most heavily heat stressed area in any rotary valve assembly of this type. Furthermore, in an inline multi-cylinder axial flow rotary valve engine there is no room to accommodate a cast cooling passage around the spark plugs and it is difficult to arrange a flow of water past the spark plug.
Adequate cooling of this area is critical for two reasons. Firstly as discussed above, the axial seal temperature must be kept at a level below that required to carbonise the lubricating oil on the surface of the seal. Secondly, the very high temperatures generated in this area when they are not adequately cooled causes localised buckling pushing the cylinder head and axial seals radially inwards towards the rotary valve with consequent seizing of the rotary valve in the cylinder head.
Rotary valve designs to date have either failed to address this problem, as evidenced by the engines disclosed in U.S. Pat. No. 4,852,532 (Bishop) and U.S. Pat. No. 4,782,801 (Ficht et al), or have chosen less optimum rotary valve arrangements that allow the spark plug to be located well away from the bore and the seal slots, such as disclosed in U.S. Pat. No. 6,321,699 (Britton). In the former case, the designs have supplied cooling water to those areas least in need of cooling and have failed to provide cooling in those areas where cooling is critical.
The engine disclosed in U.S. Pat. No. 4,852,532 (Bishop) has cooling that is typical of the prior art and it illustrates the problems associated with cooling the cylinder head of an axial flow rotary valve engine using cast cooling passages. The cooling passages in the head are cast in a similar manner to those of a typical poppet valve engine. A water jacket is also cast into the cylinder block, surrounding the cylinders. The coolant is delivered to one end of the water jacket and passes through the water jacket from one end of the engine to the other. As coolant passes through the water jacket, it is bled off into the cast cooling passages in the head through holes in the bottom of the head. The coolant then flows through the cast cooling passages between the rotary valves to the top of the head where it then flows to a single collection point. Generally, the coolant is returned to the same end of the engine as it was delivered.
In this arrangement, the coolant flow in the cylinder head to the collection point travels through cast cooling passages in the cylinder head located above the rotary valves. This has the disadvantage that these passages above the rotary valves must be of sufficient cross section to ensure that the entire engine coolant flow can pass through it without excessive pressure drop. As the maximum coolant flow rates are high, the flow area will need to be large which adds considerably to the height of the engine. Furthermore the total volume of coolant in the cooling system is larger than would otherwise be necessary. The total volume of coolant is an important issue as it determines how fast an engine reaches operating temperature from a cold start which in turn has a direct effect on exhaust emissions.
The unique difficulties of cooling axial flow rotary valve engines having a single rotary valve per cylinder compared to poppet valve engines and other types of rotary valve relate to the large diameter of the axial flow rotary valves (typically up to 75% of the cylinder bore size) and close cylinder bore spacing of modern engines leaving little room between adjacent rotary valves for cast cooling passages. Furthermore, the spark plugs must be accommodated in the limited spaces between the valves, and in order to position the spark plugs as close to the centre of the cylinder as possible, they must be positioned as close as possible to the cylinder head bores and the axial seal slots. This leaves insufficient room to cast a cooling passage between each cylinder head bore its associated spark plug. This is illustrated by FIG. 7 of U.S. Pat. No. 4,852,532 (Bishop) where there are no cooling passages between each cylinder head bore and its associated spark plug. At best, the cast cooling passages only cool approximately half the periphery of each spark plug mounting hole. Furthermore the portion of the periphery that can be cooled is subject to a much smaller heat load than the portion that cannot be cooled.
The area surrounding the axial seal slot adjacent the spark plug mounting hole is the hottest region in the cylinder head and the engine disclosed in U.S. Pat. No. 4,852,532 (Bishop) does not have any cooling in this area. Providing cast cooling passages that extend into these regions is impractical.
The problems with using cast cooling passages to cool an axial flow rotary valve engine are exacerbated by the holes connecting the cast cooling passages in the head with the cylinder block water jacket being constrained by geometry considerations to lie some distance from the centre line of the cylinders, and hence some distance from the spark plugs. The flow through these holes directs coolant towards the top of the head, rather than towards the spark plugs, and consequently the spark plugs are located in a stagnant area where there is relatively little coolant movement. This problem is further exacerbated if the holes feeding coolant to the head are located on both sides of the spark plugs, in which case coolant is fed towards the spark plugs from both sides preventing a strong flow of coolant past the spark plugs. This problem can be partially addressed by fitting baffles in the cast passages above the holes feeding coolant to the head. These baffles direct coolant flow towards the spark plugs. However, they dramatically increase the complexity of the cylinder head assembly and require access to the cast cooling passages for fitment.
The problem of cooling an axial flow rotary valve engine using cast cooling passages is made even more difficult in the case of engines having two spark plugs per cylinder. In these designs the rotary valve is typically positioned in the centre of the cylinder and the spark plugs are located on either side of the valve. This means that in a multi-cylinder engine there are two adjacent spark plugs between each pair of rotary valves, further limiting the available space around the spark plug bosses for cast cooling passages.
Other types of rotary valve, such as radial flow valves and valves having cut-outs in their periphery, can be arranged in multi-cylinder engines with their valve axis parallel to the crankshaft axis. These types of rotary valve have the advantage of simpler drive arrangement and cooling requirements compared with axial flow rotary valves but their performance is inferior. These types of valve are typically formed in one or two shafts, each extending the length of a multi-cylinder inline engine. In this case, the cast cooling passages can simply be arranged to surround the valve and extend from one end of the engine to the other in close proximity to the outside diameter of the valve, which is not possible in an axial flow rotary valve having its axis perpendicular to the crankshaft.
Rotary valve arrangements having the valve formed as a single shaft extending the length of the engine parallel to the crankshaft generally have their spark plugs placed under the valve near the periphery of the cylinder, pointing towards the side of the engine. This spark plug location is possible because, unlike a multi-cylinder axial flow rotary valve engine, there is no adjacent rotary valve to interfere with. Typical examples of this arrangement are disclosed in DE 2460164 A1 (Volkswagenwerk AG) and U.S. Pat. No. 6,321,699 (Britton). This arrangement has the advantage that the spark plugs are remote from the valve leaving ample room for generous cooling passages between the valve and the spark plugs. However, it is well known that a spark plug located adjacent the cylinder wall results in poor combustion performance.
Rotary valve arrangements having the valves formed in two shafts extending the length of the engine overcome this issue by placing the spark plug between the valves at or near the cylinder centre, which is the optimum position from a combustion perspective. A typical example of this arrangement is shown in U.S. Pat. No. 4,949,685 (Doland et al). In this type of arrangement, the axis of the valves is generally well outside the cylinder leaving ample room to cast cooling passages between the valves and the spark plug. However, this type of arrangement has the obvious disadvantage of requiring two valves instead of one.
The present invention seeks to provide a cooling system for axial flow rotary valve engines that ameliorates one or more of the disadvantages associated with the prior art.